The destructive effect of visible light and certain dyes on biological systems of all types is well-known (Photodynamic Effect). In addition, some of the same dyes which are highly effective in such reactions, have been repeatedly shown to be preferentially retained in malignant tumors in animals and humans. Several such dyes have been investigated as aids in cancer diagnosis and tumor delineation, but until very recently, essentially no attempts have been made to test the therapeutic potential of exposing these dye-containing tumors to visible light. It is our purpose to test the feasibility of such a therapy for malignant tumors. We will examine the critical variables of this process with tumor cells "in vitro" and "in vivo" with solid implants of various tumors. Dyes to be tested include those previously shown to accumulate specifically in malignant tissue, e.g., fluorescein and hematoporhyrin. The optimum dye, concentration, type and degree of light exposure, effect of tumor types and degree of visible light penetration into mouse tissue will be defined. Efficiencies of dyes in maximizing tumor response will be determined using both photochemical and tumor cell viability methods. Tumor response will be measured by tumor volume changes and cell viability. Measurements of monochromatic light penetration in tissue will be carried out using appropriate measuring equipment.